We are grateful to Todd for his comments and observations. He specifically asks about the numbers of patients in the two groups and the overlap, group 1 being patients undergoing emergency MRI for suspected cauda equina syndrome and group 2 being patients having emergency discectomy for clinically and radiologically confirmed cauda equina syndrome. The confusion perhaps relates to ambiguity over the term ‘emergency’. We have studied group 1 as defined as patients having MRI out of normal working hours for suspected cauda equina syndrome, hence an emergency group. This will, however, miss a small number of patients from the local area who underwent an emergency MRI for suspected cauda equina syndrome during working hours using our own MRI scanner. The second group of patients we have defined as those undergoing unscheduled (i.e. not admitted electively) discectomy for cauda equina syndrome, some being done during normal working hours. True, 27 of the 82 patients having emergency (out-of-hours) MRI proceeded to surgery, but in only 5 of these 27 was the clinical and radiological correlate sufficient to warrant direct procession to surgery that night, hence a true ‘emergency’ case of cauda equina syndrome. This represents a difficulty of this retrospective study but, moreover, a difficulty which confounds much of the literature: specifically what does constitute the cauda equina syndrome? Certainly, the combination of perineal sensory loss, painless urinary retention and/or incontinence with unilateral or bilateral L5 or S1 symptoms as a consequence of compressive pathology in the spinal canal, typically a large disc prolapse, will qualify. However, the less clearly defined group of incomplete cauda equina syndrome patients are defined differently by different commentators.

The work presented represents a documentation of current practice and allows the assessing clinician and the on-call spinal service to understand what index of suspicion should be maintained for true cauda equina syndrome in this group of patients, prior to interhospital transfer for on-call MRI. It is strengthened by reading alongside the contemporary study by Bell et al.,1 who found positive predictive values for a large central disc of around 0.6 for each of urinary retention, urinary frequency, urinary incontinence, altered urinary sensation and altered perineal sensation. Todd' proposed algorithm for attempting to identify the most urgent cauda equina syndrome patients from those who may reasonably be imaged the next day is a good proposition and it would be interesting to see it applied hypothetically to a cohort of well-documented patient episodes to test its strengths and weaknesses.

We agree that the timing of surgery is a matter of debate; we also agree, however, that, given the good outcomes seen following expeditious surgery for incomplete cauda equina syndrome, to treat this as a non-emergency matter falls foul not only of the somewhat weak published evidence but, more seriously, of medical and scientific common sense. We agree that it is appropriate to offer such emergency treatment to patients with complete cauda equina syndrome, especially given that long delays from the onset of bladder paralysis from cauda equina syndrome are fortunately uncommon. The issue of surgery as an emergency is indeed controversial; however, if a large central disc prolapse is suspected to be causing impending or true bladder disturbance, the risk–benefit analysis, even in the absence of good quality evidence, surely falls strongly in favour of decompressive surgery. A further confounding factor in this situation is the use of unreliable outcome measures that are highly specific to the individual: arguably, a bilateral foot drop will be a greater inconvenience to some patients than loss of sexual function or the requirement to use a catheter. The difficulties of outcome assessment have been discussed previously and are likely to continue.2

There is unlikely to be good quality prospective scientific evidence in this area given the impossibility of developing a realistic and predictable animal model of disc prolapse, and equally given the inappropriateness of subjecting patients with such a critical condition to a trial of delayed versus immediate surgery. Hence, the modern literature is likely to be confined to observational studies with the opinions of experienced clinicians.